This invention relates to a piston, in particular of light metal, for internal combustion engines having substantially circular sleeves, comprising a piston skirt substantially closed along its periphery, pin bosses set back with respect to the piston skirt diameter in the direction towards the piston axis to the extent that the spacing between the outer pin boss surfaces corresponds to 60 to 80% of the piston diameter. Two supporting skirt wall sections remain on the piston skirt on the thrust and counter-thrust side and receive the transverse forces. The supporting skirt wall section on the thrust-side is larger at least in the peripheral direction of the skirt than the supporting skirt wall section on the counter-thrust side. Two wall sections connect the supporting skirt wall sections which are continuously convexly curved outwards along the piston skirt periphery at least at the open end of the skirt.
Such pistons are known from Japanese utility model publication JP-A-64-3054.They are the result of a long development. In order to fulfill the general motor - technical development aims of:
High power compression corresponding to high revolutions of the motor, PA0 quiet running of the pistons, and PA0 low power loss due to friction between the piston and the cylinder, PA0 maintain the piston weight as low as possible by means of far-reaching reduction in the wall thickness; PA0 optimize the supporting skirt surfaces of the piston, responsible for receiving the transverse forces, to the extent that they are not larger than necessary, in order to prevent a forcing through of the oil film due to the effect of the specific surface thrust, and to the extent that they are as small as possible in order to maintain as low as possible the shear forces which are constantly to be overcome in the oil film; and PA0 reduce the play between the piston and cylinder wall to the greatest extent possible in order to limit rocking movements of the piston and deflection noises.
it has already been attempted for a long time to:
In proceeding towards these aims, the piston skirt was initially developed from a cylindrical to an oval, crowned outer shape in the sense that the skirt was reduced to a small extent of under 1 mm in the direction of the pin axis from the contact point on the cylinder wall in order to allow the piston to elastically mitigate dimensional overlapping due to thermal expansion in the connecting rod pivoting plane. On account of this measure, the assembly play perpendicular to the gudgeon pin axis could be reduced.
Further development towards always greater reduction of the piston skirt lead to the so-called box- or H-piston, which only had somewhat cylindrical skirt regions as supporting surfaces in the thrust and counter-thrust directions, but, contrary to this, almost straight wall regions in the pin direction which are greatly set back towards the piston central axis. On account of this measure, not only the frictional power losses caused by the contacting skirt surfaces were further reduced; a substantial saving in weight was achieved together with the shortening of the piston pin (c.f. EP-B-0 171 825, in particular FIG. 3).
In order to take the various loading ratios of the piston on the thrust and counter-thrust sides into account, it is additionally known to form the supporting surface on the thrust side, at least in the peripheral direction of the piston skirt, to be larger than the supporting surface on the counter-thrust side (c.f. EP-B-0 171 825, particularly FIG. 4).
The deficiency of these pistons according to the described state of the art results from the selected form or the selected shape of the wall sections which connect the supporting wall sections of the piston skirt with one another, as well as the type of joining of these wall sections to the sections of the piston skirt forming the supporting surfaces. In the known pistons, these connecting wall sections extend substantially linearly and perpendicularly to the piston axis or radially to the joining point at the cylindrical supporting wall section, thus providing the piston skirt with a large stiffness in the direction of the connecting rod pivoting plane, i.e. in the region of the supporting surface joining point. Furthermore, the transition from the straight wall sections into the cylindrical supporting surfaces with only one, mostly small radius of curvature results in a zone of high stress concentration.
On account of the continuously necessary elastic cushioning of the supporting skirt walls during operation of the motor, material cracks are easily generated in these critical transition zones. In the state of the art, these possible cracks are countered with material additions or rib-like reinforcements (see again EP-B-0 171 825, particularly FIGS. 3 and 4), which however, increase stiffness of the piston skirt and thus the risk of piston scuffing, and also work against the savings in weight that are sought.
In the piston according to Japanese utility model JP-A-64-3054 already described and forming the basis of the prior art, the supporting skirt wall section is not only greater on the thrust side in the pheripheral direction of the skirt than the supporting skirt wall section on the counter-thrust side as in the known piston already discussed (EP-B-0 171 825, FIG. 4), but the wall sections connecting the supporting skirt wall sections are also at least convexly curved outwardly in the region of the skirt beneath the pin boss. However, the transition regions between the supporting skirt wall sections and the connecting wall sections as well as between these and the pin bosses are still characterized by abrupt changes in direction, which results in the consequence that stress peaks occur there even though this piston is more elastic than the previously revealed type of piston on account of the convexity of the connecting wall sections.
Due to the fact that the pin bosses project radially outwardly with respect to the connecting wall sections, shorter supporting skirt wall sections result in the peripheral direction for given pin lenghts than when the outer surfaces of the connecting wall sections pass over into the outer pin boss surfaces in a flush manner, as in the known box piston (cf. EP-B-0 171 825, particularly FIG. 3). Therefore, the known piston has a higher surface compression.
Finally, a piston is described in the older but not pre-published EP-Al-0 430 362, the connecting walls sections of which extend convexly outwardly between the supporting skirt wall sections over the entire length of the shaft; however, this piston is symmetrically formed, ie. both supporting shaft wall sections are the same in the peripheral direction.
The technical problem with respect to this prior art forming the basis of the present invention is to provide a piston which ensures a high elasticity of the entire piston skirt when using a light mode of construction and in which no stress concentrations arise.